Isolation and enrichment of cells/micro-particles from a biological sample is one of the first crucial processes in many biomedical and homeland security applications [1]. Water quality analysis to detect viable pathogenic bacterium [2-6] and the isolation of rare circulating tumor cells (CTCs) for early cancer detection [7-19] are important examples of the applications of this process. Conventional methods of cell concentration and separation include centrifugation, filtration, fluorescence activated cell sorting, or optical tweezers. Each of these techniques relies on different cell properties for separation and has intrinsic advantages and disadvantages. For instance, many of the known techniques require the labeling or tagging of cells in order to obtain separation. These more sensitive techniques may require prior knowledge of cell-specific markers and antibodies to prepare target cells for analysis.
Dielectrophoresis (DEP) is the motion of a particle in a suspending medium due to the presence of a non-uniform electric field [28, 29]. DEP utilizes the electrical properties of the cell/particle for separation and identification [29, 30]. The physical and electrical properties of the cell, the conductivity and permittivity of the media, as well as the gradient of the electric field and its applied frequency are substantial parameters determining a cell's DEP response.
The application of dielectrophoresis to separate target cells from a solution has been studied extensively in the last two decades. Examples of the successful use of dielectrophoresis include the separation of human leukemia cells from red blood cells in an isotonic solution [7], entrapment of human breast cancer cells from blood [8], and separation of U937 human monocytic from peripheral blood mononuclear cells (PBMC) [9]. DEP has also been used to separate neuroblastoma cells from HTB glioma cells [9], isolate cervical carcinoma cells [10], isolate K562 human CML cells [11], separate live yeast cells from dead [12], and segregate different human tumor cells [13]. Unfortunately, the microelectrode-based devices used in these experiments are susceptible to electrode fouling and require complicated fabrication procedures [33, 34].
Insulator-based dielectrophoresis (iDEP) has also been employed to concentrate and separate live and dead bacteria for water analysis[2]. In this method, electrodes inserted into a microfluidic channel create an electric field which is distorted by the presence of insulating structures. The devices can be manufactured using simple fabrication techniques and can be mass-produced inexpensively through injection molding or hot embossing[35, 36]. iDEP provides an excellent solution to the complex fabrication required by traditional DEP devices however, it is difficult to utilize for biological fluids which are highly conductive. The challenges that arise include joule heating and bubble formation[37]. In order to mitigate these effects, oftentimes the electrodes are placed in large reservoirs at the channel inlet and outlet. Without an additional channel for the concentrated sample[36], this could re-dilute the sample after it has passed through a concentration region.
While many have had success designing and fabricating different DEP and iDEP microdevices to manipulate particles in biological fluids, there are some potential drawbacks of these techniques. The traditional DEP technique suffers from fouling, contamination, bubble formation near integrated electrodes, low throughput, and an expensive and complicated fabrication process [33, 34]. The insulating obstacles employed by iDEP are meant to address these shortcomings and are less susceptible to fouling than integrated electrodes [38]. The iDEP fabrication process is also much less complicated; the insulating obstacles can be patterned while etching the microchannel in one step. This technique has the added benefit of making the process more economical in that mass fabrication can be facilitated through the use of injection molding. Unfortunately, one of the primary drawbacks of an iDEP system is the presence of a high electric field intensity within the highly conductive biological fluid inside the microchannel [33, 39]. The relatively high electrical current flow in this situation causes joule heating and a dramatic temperature increase. The ideal technique would combine the simple fabrication process of iDEP and resistance to fouling with the reduced susceptibility to joule heating of DEP while preserving the cell manipulation abilities of both methods.